Synchronous control system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a synchronous control system for effecting a synchronous and superimposed operation among different channels of a multi-channel-control machine tool, and more specifically, to a synchronous control system wherein the reliability of the system is improved when the synchronous and superimposed operation is effected. 
     2. Description of the Related Art 
     Numerically controlled machine tools, and in particular numerically controlled lathes and the like, are provided with a multiplicity of tool posts to enable a workpiece to be cut by a plurality of tools, to thereby increase the cutting speed. Accordingly, some of the tool posts must effect not only a movement relative the movement of a spindle but also a movement including the amount of movement of the spindle. This operation is referred to as a superimposed operation. 
     FIG. 3 is a schematic block diagram of a numerically controlled lathe as a multi-channel-control machine tool provided with a plurality of tool posts, wherein 61 designates a spindle controlled in a Z direction, herein referred to as a Z1-axis. In the numerically controlled lathe, the Z-axis is operated synchronously with the rotation of the spindle 61. 
     In FIG. 3, 62 designates a first tool post controlled only in an X direction, herein referred to as an X1-axis. Note, the spindle 61 and the first tool post 62 constitute a usual X - Z coordinate. 
     Designated at 63 is a second tool post controlled in the X- and Z-axis directions, herein referred to as an X2-axis and a Z2-axis. The second tool post 63 is independently operated on the X - Z coordinate when the spindle 61 does not move along the Z1-axis. However, when the spindle 61 is moved in the Z1-axis direction, the operation of the Z2-axis is superimposed with that of the Z1-axis of the spindle 61, since the movement of the Z2-axis must include the amount of movement of the Z1- axis. More specifically, the Z2-axis must be moved by an amount determined by adding the amount of movement of the Z1-axis to the original amount of movement of the Z2-axis. 
     Designated at 64 is a third tool post controlled only in the Z-axis direction, herein referred to as a Z3-axis. The operation of the Z3-axis is also superimposed with that of the Z1-axis. 
     When the operation of the multi-channel-control machine tool is superimposed as described above, each coordinate axis is controlled by a respective servomotor, but since a conventional servomotor control system can not detect a relative follow-up error therebetween, an alarm condition cannot be detected even if the control of the servomotors is erroneously effected. 
     More specifically, although servomotor control circuits for controlling the Z1-axis and the Z2-axis in FIG. 3 independently monitor an excessive follow-up error wherein a follow-up error amount exceeds a predetermined amount, they can not monitor a relative follow-up error amount or the like. 
     SUMMARY OF THE INVENTION 
     Taking the above into consideration, an object of the present invention is to provide a synchronous control system wherein the reliability of the system is improved when a synchronous and superimposed operation is effected. 
     To solve the above problem, in accordance with the present invention, there is provided a synchronous control system for effecting a synchronous and superimposed operation among different channels of a multi-channel-control machine tool, wherein a third follow-up error amount of a servomotor responsive to a superimposing instruction is simulated, a difference is determined by subtracting the third follow-up error amount to the superimposing instruction from a second follow-up error amount of a servomotor for a superimposed operation axis. The difference is then compared with a first follow-up error of a servomotor for a synchronous axis to produce an alarm condition when the comparison indicates the operation of the servomotors is synchronized and superimposed. 
     An amount of movement of the synchronous operation axis is superimposed on a servomotor control circuit of the superimposed operation axis. Thus when a follow-up error amount of the independent amount of movement of the superimposed operation axis is simulated and the value obtained as a result of the simulation is subtracted from a follow-up error amount of the servomotor control circuit of the superimposed operation axis, a follow-up error amount of the synchronous operation axis is determined. 
     If the superimposed operation axis and the synchronous operation axis are operating normally, the follow-up error amount determined as above must coincide with the follow-up error amount of the servomotor control circuit of the synchronous operation axis. Thus, a comparison of both amounts provides a confirmation of a normal operation of the synchronous operation axis and the superimposed operation axis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of control effected by a synchronous control system according to the present invention; 
     FIG. 2 is a detailed diagram of a simulation circuit; and 
     FIG. 3 is a schematic diagram of a numerically controlled lathe as a multi-channel-control machine tool provided with a plurality of tool posts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the present invention will be described below with reference to drawings. 
     FIG. 1 is a block diagram of control effected by a synchronous control system according to the present invention. 
     In this description, only the control of the Z1-axis (synchronous operation axis) and the Z2-axis (superimposed operation axis) will be described, in order to facilitate understanding. However, the approach is the same for other synchronous and superimposed operation axes. 
     In FIG. 1, 10 designates a servomotor control circuit for effecting a speed control and current control of the Z1- and Z2-axes. The servomotor control circuit 10 comprises (although not shown) a microprocessor, a digital signal processor (DSP) or the like, and accordingly, the following transfer function (discussed below) is processed by software in the microprocessor or the DSP. 
     In FIG. 1, 11 designates an adder for adding a movement instruction Z1C to the Z1-axis and a movement instruction Z2C to the Z2-axis, 12 designates an adder for subtracting a positional feedback amount from the movement instruction output from adder 11 and outputting the result of the subtraction, where the output from the adder 12 is a follow-up error amount ε2 of the Z2-axis, 13 designates a gain, and 14 designates a speed and current control circuit for effecting the speed control and the current control. 
     Designated at 15 is a simulation circuit for simulating a follow-up error amount of a movement, instruction given to the Z2-axis (superimposed operation axis), to independently move the axis and to output a follow-up error amount ε3 when the Z2-axis is independently moved. 
     Designated at 16 is an adder for subtracting a positional feedback amount of the Z1-axis from the movement instruction Z1C to the Z1-axis and outputting the result of the subtraction. The output ε1 from the adder 16 being, a follow-up error amount of the Z1-axis. Designated at 17 is a gain, and designated at 18 is a speed and current control circuit for effecting a speed control and a current control. 
     Designated at 19 is an adder for subtracting the follow-up error amount ε3 of the Z2-axis produced when the Z2-axis is independently moved from a follow-up error amount ε2 of the Z2-axis, and outputting the result of the subtraction. Designated at 20 is a comparator for comparing the output from the adder 19 with the output from the adder 16 for the Z1-axis (i.e. ε1). When the Z1-axis and the Z2-axis are normally operated, the inputs to the comparator 20 should coincide, and therefore, when a difference between both inputs exceeds a predetermined value, an alarm condition is produced. Since the value ε3 is determined as a result of the simulation, the inputs to the comparator do not strictly coincide and thus a difference within the predetermined value is ignored as an allowance. 
     Designated at 31 is a servo amplifier of the Z2-axis and designated at 32 is a servomotor thereof for controlling the movement of the mechanical unit of the Z2-axis, i.e., the tool post 63 in FIG. 3, in the Z-axis direction. 
     Designated at 41 is a servo amplifier of the Z1-axis and designated at 42 is a servomotor thereof for controlling the movement of the mechanical unit of the Z1-axis, i.e., the spindle 61 in FIG. 3, in the Z1-axis direction. 
     FIG. 2 shows the details of the simulation circuit 15, wherein 51 designates an adder, 52 designates a gain, and 53 designates an integrator. The output from the integrator 53 is negatively fed back to the adder 51. 
     With the arrangement as described above, a transfer function of the simulation circuit 15 of FIG. 3 can be determined as shown in equation 54 illustrated in FIG. 2. 
     As described above, according to the present invention, a follow-up error amount of an independent movement of the superimposed operation axis is simulated and the follow-up error amount determined as a result of the simulation is subtracted from a follow-up error amount of the servomotor control circuit of the superimposed operation axis to determine a follow-up error amount of the synchronous operation axis. The follow-up error amount thus determined is compared with an actual follow-up error amount of the synchronous operation axis. As a result, a synchronous error produced in a superimposed operation, which error would not otherwise be detected, can be detected. Thus, the reliability of a multi-channel-control machine tool is improved.